The triboelectric effect is based on contact-induced electrification in which a material becomes electrically charged after it is contacted with a different material through friction. It has been proposed to make use of this charge flow to power mobile devices such as sensors and smartphones by capturing the otherwise wasted mechanical energy from such sources as walking, the wind blowing, vibration or ocean waves. Beyond the use as a power source, the triboelectric effect has been proposed for sensing without an external power source. Because the generators produce current when they are perturbed, they can be used to measure changes in flow rates, sudden movement, or even falling raindrops.
The triboelectric effect is based on a series that ranks various materials according to their tendency to gain electrons (become negatively charged) or lose electrons (become positively charged). This series is for example disclosed in A. F. Diaz and R. M. Felix-Navarro, A semi-quantitative tribo-electric series for polymeric materials: the influence of chemical structure and properties, Journal of Electrostatics 62 (2004) 277-290. The best combinations of materials to create static electricity are one from the positive charge list and one from the negative charge list (e.g. PTFE against copper, or FEP against aluminium). Rubbing glass with fur, or a comb through the hair are well-known examples from everyday life of triboelectricity.
In its simplest form, a triboelectric generator thus uses two sheets of dissimilar materials, one an electron donor, the other an electron acceptor. When the materials are in contact, electrons flow from one material to the other. If the sheets are then separated, one sheet holds an electrical charge isolated by the gap between them. If an electrical load is then connected to two electrodes placed at the outer edges of the two surfaces, a small current will flow to equalize the charges.
By continuously repeating the process, an alternating current can be produced. In a variation of the technique, the materials—most commonly inexpensive flexible polymers—produce current if they are rubbed together before being separated. Generators producing DC current have also been proposed. The volume power density may reach more than 400 kilowatts per cubic meter at an efficiency of more than 50%.
The power output can be increased by applying micron-scale patterns to the polymer sheets. The patterning effectively increases the contact area and thereby increases the effectiveness of the charge transfer.
Recently an emerging material technology for power generation (energy harvesting) and sensing has been developed which makes use of this effect as disclosed in Wang, Z. L., “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”. ACS Nano: 131014091722005. doi:10.1021/nn404614z, 2013.
Based on this effect several device configurations have been developed of so-called triboelectric generators (“TEG”). Some devices operate in a contact mode, and others operate in a friction mode.
One configuration has been developed specifically for power generation from a shoe insole. This generates power from the steps taken by a user, and the generated power may for example be used for charging of mobile portable devices. The device comprises a multiple layer structure formed on a zig-zag shaped substrate. The device operates based on surface charge transfer due to contact electrification. When a pressure is applied to the structure, the zig-zag shape is compressed to create contact between the different layers, and the contact is released when the pressure is released. Details can be found in the article “Integrated Multilayered Triboelectric Nanogenerator for Harvesting Biomechanical Energy from Human Motiosn” of Peng Bai et. al. in ACS Nano 2013 7(4), pp 3713-3719.
Instead of using a contact and non-contact mode of operation, a TEG can operate in a sliding mode. A design which enables energy to be harvested from sliding motions is disclosed in the article “Freestanding Triboelectric-Layer-Based Nanogenerators for Harvesting Energy from a Moving Object of Human Motion in Contact and Non-Contact Modes” in Adv. Mater. 2014, 26, 2818-2824. A freestanding movable layer slides between a pair of static electrodes. The movable layer may be arranged not to make contact with the static electrodes (i.e. at small spacing above the static electrodes) or it may make sliding contact.
Another configuration that has been developed is a rotational disc TEG which can be operated both in contact or non-contact mode. Rotational disc TEGs typically consist of at least one rotor and one stator each formed as set of spaced circle sectors. The sectors overlap and then separate as the two discs rotate relative to each other. In such a rotating disc triboelectric generator, electricity is generated by the combinations of two main physical mechanisms: coupling between contact electrification (triboelectric charging) and rotational electrostatic induction (in-plane charge separation due to redistribution of electrical charges caused by the influence of nearby charges)
The limitations of early versions of segmentally structured disc TEGs were that the rotational and stationary triboelectric layers require deposition of metal electrodes and connection with electrical leads, leading to inconvenient operation of the rotational part. Furthermore intimate contact is mandatory to achieve efficient electricity generation, which results in possible material wear, wear particles, instability of output, and generally limited lifetime of the TEG.
A disk TEG with both groups of patterned electrodes attached onto a stationary disk, together with a freestanding triboelectric layer on a rotational disk can resolve these issues, as disclosed in Long Lin et al., Noncontact Free-Rotating Disk Triboelectric Nanogenerator as a Sustainable Energy Harvester and Self-Powered Mechanical Sensor. ACS Appl. Mater. Interfaces, 2014, 6 (4), pp 3031-3038.
With such a structure, there is no necessity for electrode deposition or electrical connection for the rotational part, which dramatically improves the operating facility of the energy harvester. Moreover, owing to the unique feature of this new electricity-generation mechanism, the non-contact free-rotating disk triboelectric nanogenerator (FRD-TEG) can be operated without friction after initial contact electrification, with little loss in performance but superior durability, because the surface triboelectric charges are preserved on insulator surfaces for hours.
There are still further designs of triboelectric generator, such as a double-arch shaped configuration based on contact electrification. A pressure causes the arches to close to make contact between the arch layers, and the arches returns to the open shape when the pressure is released. A triboelectric nanogenerator has also been proposed which is formed as a harmonic resonator for capturing energy from ambient vibrations.
The TEG's can for example generate triboelectricity up to area power densities levels of 670 W/m2.
It will be clear that there are many different designs of TEMG device, each tailored to a specific mode of operation. Some examples are outlined and referenced above. In general, four different general modes of operation may be identified.
A first mode is a vertical contact-separation mode, in which two or more plates are brought into and out of contact by an applied force. This may be used in shoes, with the contact resulting from the pressure applied by the user stepping. The zig-zag shaped arrangement described and referenced above is an example.
A second mode is a linear sliding mode in which plates are made to slide relatively to each other to change the area of overlap. A rotational disk TEG discussed above is an example. This may be used in a wave energy harvesting system.
A third mode is a single electrode mode in which one surface is for example grounded for example a floor or road, and the movement influences only one layer.
A fourth mode is a freestanding triboelectric layer mode, which is designed for harvesting energy from an arbitrary moving object to which no electrical connections are made. This object may be a passing car, a passing train, or a shoe.
Triboelectric generators are designed to generate power very briefly in response to an applied motion. The character of the motion affects the voltage or current generated, and the optimal load to absorb the energy will vary with the motion.